Adeno-associated virus (AAV), a defective human parvovirus, is one of the smallest DNA containing viruses known. Although more than half of the adult population has neutralizing antibody to AAV, there is no known pathology associated with AAV infection. However, AAV does readily establish persistent infections both in vivo and in vitro. Evidence suggests that virus persistence is at the level of AAV DNA which, at least in vitro, can integrate into cellular DNA. Integration of AAV DNA is probably site-specific for the AAV terminal sequences. The ends of AAV DNA have both regular and inverted repeats and are analogous to the terminal structures of prokaryotic and eukaryotic transposable elements and the long terminal repeats of retroviruses. These elements have been shown to integrate into host DNA and have the potential for causing mutation. In addition infectious AAV can be rescued in vitro from latently infected cells following superinfection with a helper adenovirus. These findings suggest that AAV might be a unique non-pathogenic integrative eukaryotic vector especially useful for studies of eukaryotic gene regulation, isolation of selectable genes form libraries, and eventually gene therapy. We will develop vectors which will contain specific eukaryotic genes bounded by one or both of the AAV termini. These vectors will be tested to determine their efficiency of transformation, stability of transformation, and whether the vector DNA is integrated or episomal in the recipient cells. They will be compared with similar vectors lacking the AAV termini. If, as expected, the AAV vector does promote the integration of cloned genes into the genome of recipient cells, the ability of superinfection with helper Adenovirus (Ad) to rescue the vector sequences from these transformed cells will be tested. Similarly it will be determined whether genes containing fewer than 4,000 nucleotides can be packaged into AAV virions following superinfection with helper Ad and AAV.